1. Field of Invention
The present invention relates to a light source device lighted by means of a laser beam emitted from a laser device, which is ideally suited for use in an exposure device.
2. Description of Related Art
Light source devices which are configured such that a laser beam from a laser device is radiated towards a light emission tube in which a light emitting gas is enclosed, the gas is excited and light is emitted are known (see JP-A-61-193358 (1986)). With the device disclosed in JP-A-61-193358 (1986), a beam from a laser oscillator oscillating continuous or pulsed laser light is focused by a focusing component of an optical system such as a lens and radiated towards a light emission tube in which a light emitting gas (light emitting means) is enclosed. The light emitting gas in the light emission tube is excited and light is emitted. In lines 16 to 18 from the top of the right upper column on page 2 of JP-A-61-193358 (1986) it is stated that said laser oscillator oscillates continuous or pulsed laser light having sufficient intensity for the discharge excitement of the enclosed gas.
As to the laser beam exciting the light emitting gas enclosed in the light emission tube a continuous or pulsed laser beam is contemplated, as is stated in JP-A-61-193358 (1986), but is has been found that the following problems occur irrespective of whichever laser beam is used.
(a) As in the case of a pulsed laser beam the lighting is started by ‘oscillating a pulsed laser beam having sufficient intensity for the discharge excitement of the enclosed gas’, the laser beam is radiated towards the enclosed gas intermittently, as is shown in FIG. 18(a), because of which the high-temperature plasma state may become interrupted with the interruption of the laser light and it is difficult to maintain the high-temperature plasma state at all times. In other words, there is the problem that the discharge maintenance is instable.(b) When, in the case of a continuous laser beam, a continuous laser beam having sufficient intensity for the discharge excitement of the enclosed gas is oscillated, the lighting is started, but if the same energy as at the time of the start of the lighting is inputted also when the high-temperature plasma state is maintained, as is shown in FIG. 18(b), the tube sphere is heated, and because of this heat distortions of the tube sphere are generated and breakages occur. In other words, there is the problem that the lighting life cycle is short. Further, the power of the laser light necessary to start the discharge amounts from some ten to some hundred kW, but a laser device continuously outputting laser light with such a high output is large and costly and therefore impractical.